1. Field of the Invention
The present invention relates to a technology for improving utilizaton efficiency of a radio channel for random access communication in a mobile communication system.
2. Description of the Related Art
Nowadays, a major shift in the way that information is transmitting through a mobile communication system such as a cellular phone system is the shift from speech to packet data. This is because cellular phones are provided with higher functionality, and functions such as site browsing, e-mail, camera, music/video reproduction have been successively added so that large-volume data associated with these functions is transmitted through networks.
In order to transmit such packet data from a mobile station such as a cellular phone to a base station, many mobile communication systems use a random access communication scheme. The random access communication scheme is suitable for a communication which does not have information to be continuously transmitted such as speech calls.
The random access communication scheme shares one radio channel determined by frequency, time slot, spreading code or the like among a plurality of mobile stations (e.g., see JP-A-1996-181645, JP-A-1997-200848, JP-A-1998-173584 and JP-A-2005-101990).
Therefore, to prevent a specific mobile station from occupying a radio channel for a long time, when the size of random access data is large, the mobile station divides the data into portions of a predetermined data length and transmits the divided random access data to the base station after an interval.
For example, a mobile communication system based on a W-CDMA (Wideband Code Division Multiple Access) scheme is provided with a radio channel called PRACH (Physical Random Access Channel) as a radio channel for random access communication.
Here, the random access communication method in a related mobile communication system will be explained with reference to FIG. 1 and FIG. 2.
Referring to FIG. 1 and FIG. 2, when random access data to be transmitted to a base station is produced in step 801, a mobile station sends a transmission start request signal prearranged with the base station to the base station before transmitting random access data in step 802.
When the transmission start request signal is received from the mobile station in step 803, the base station judges whether or not to permit transmission of the random access data in step 804 and transmits a transmission start permission signal or a transmission start rejection signal to the mobile station according to the judgment result in steps 805 and 806.
When a transmission start rejection signal is received from the base station (in the case of situation [A] in FIG. 1) in step 807 or when a transmission start permission signal cannot be received from the base station after a lapse of a predetermined time in step 809, the mobile station repeatedly transmits a transmission start request signal to the base station until it can receive a transmission start permission signal.
On the other hand, when a transmission start permission signal is received from the base station in step 808 (in the case of situation [B] in FIG. 1), the mobile station starts transmission of random access data of a predetermined data length to the base station in step 810. The base station performs reception processing on the random access data in step 811.
When the transmission of the random access data of the predetermined data length is completed in step 812, the mobile station stops the transmission temporarily in step 813. In that case, when there still remains some random access data in the mobile station in step 814, the mobile station repeatedly transmits a transmission start request signal to the base station until it can receive a transmission start permission signal again from the base station.
Hereinafter, the above described processing is repeated and transmission of the random access data from the mobile station is completed when there remains no more random access data in step 815.
In the above described random access communication method, the mobile station and the base station must communicate about permission or rejection for a transmission start every time the mobile station transmits random access data of a predetermined data length to the base station. However, there can also be cases where no mobile station is transmitting random access data while these exchanges are being carried out.
However, in order to efficiently transmit the increasing amount of packet data daily, it is necessary to efficiently use a radio channel for random access communication and to reduce the time during which no mobile station is transmitting data as much as possible. Furthermore, mobile stations also require that random access data be transmitted as early as possible.